SILICIFICATION. 393 



Other period. The Post-Neocomian silicification altered a portion of tlio 

 shales to jaspery masses or phthanite and formed in these and other rocks 

 innumerable minute veins of quartz. Tiie silicification attendant upon ore 

 deposition at a much later date resulted in 'the formation of great quantities 

 of opal, accompanied by a small amount of crystalline silica. The opal is 

 usually colored, and, as seen in hand specimens, is often deep black, so that 

 it considerably resembles some varieties of obsidian. It occurs in meta- 

 morphosed rocks sometimes as small spots and, again, near ore bodies in 

 large qmintities. It is much more frequent in the mines to the north of 

 San Francisco than to the south, though few deposits are unaccompanied 

 by small quantities of this material. I am not aware that it is found any- 

 where far from known traces of cinnabar and I look upon it as an indica- 

 tion of the probable presence of quicksilver wherever found. Its intimate 

 relation with metalliferous solutions is shown by the fact that it is seldom 

 if ever free from sulphides of iron or nickel, which may sometimes be seen 

 under the microscope when they are macroscopically invisible. The opal is 

 seldom absolutely free from quartz and chalcedonite, and sometimes a small 

 amount of carl)onates appears in it. Tlie chalcedonite and quartz microlites 

 in the opal are not infrequently radially arranged, forming globides which 

 dot the entire field Sometimes a perfect net of minute bands of quartz 

 traverses the opal. This net has, however, nothing to do with serpentine, 

 for, while net structure does not occur among the serpentines of the quick- 

 silver belt, it is manifest under the microscope that this quartz net repre- 

 sents an infiltration into fissured opal. The opal usually carries a few fluid 

 inclusions and microlites of an indeterminable character. 



Some of the opal or opaline chalcedon)- has certainly been deposited in 

 pre-existing openings, but a large part of it must have been deposited b}- sub- 

 stitution for rocks. This conclusion was drawn from ol)servation in the mines, 

 where the shape of the opaline masses and the manner in wliicli it was min- 

 gled with country rock, particularly serpentine, forbade the supposition that 

 it had filled cavities or fissures. Many slides present no evidence of pseudo- 

 morphism, being entirely occupied by opal, with trifling admixtures of 

 quartz etc. Others, however, show clear transitions to serpentine and, in 

 particular, distinct remnants of the grate structure so characteristic of the 



